Question: The scale on a map is 4cm : 5km. If two cities are 12cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 4cm on the map corresponds to an actual distance of 5km. A distance of 12cm on the map is the same as 3 $\cdot$ 4cm. The actual distance between the two cities is 3 $\cdot$ 5km, or 15km.